1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses of the inner focusing type. Still more particularly, it relates to wide angle high-range zoom lenses with a zoom ratio of 3.7 and a widest angular field of about 62.degree. suited to be used in photographic cameras, video cameras or cameras for broadcast.
2. Description of the Related Art
For the photographic camera and video camera, there have been a wide variety of previous proposals for zoom lenses having the focusing provision made not in the first lens group, when counted from the object side, but in another lens group which lies in the middle of the lens system, or for employing the so-called inner focusing method.
In general, the inner focusing type of zoom lens has merits over the front focusing type of zoom lens in that, owing to the smaller diameter of the first lens group, the bulk and size of the entire lens system can be easily minimized, that the focusing range can be extended to closer object distances so that particularly extreme close-up photography becomes easy to carry out, and further that, with the help of a relatively small size and light weight of the focusing lens group, a weak driving power suffices for rapid focusing so that in application to the camera of automatic focus detection, the control of focusing becomes easy.
Such an inner focusing type of zoom lens is exemplified in, for example, Japanese Laid-Open Patent Application No. Sho 63-44614 where, in the order from the object side, a first lens group of positive power, a second lens group of negative power for varying the image magnification, a third lens group of negative power for compensating for the image shift with variation of the image magnification, and a fourth lens group of positive power, or four lens groups in all, are used to form a so-called 4-group zoom lens wherein the third lens group is made to move for focusing purposes. With this zoom lens, however, a surplus space must be formed to assure the movement of the third lens group. Therefore, its total length tends to increase largely.
In general, when the inner focusing method is employed in the zoom lens, merits such as those of, as has been described before, reducing the size of the entirety of the lens system and becoming capable of rapid focusing, and further making it easy to extend the focusing range can be obtained.
However, its counterparts are to increase variation of aberrations with focusing, thus giving rise to a problem that it becomes very difficult to obtain high grade optical performance on an infinitely distant object to a close object, or the entire range of object distances, while still affording a minimization of the size of the entire lens system.
Particularly when the widest angular coverage and the range of the zoom lens increase at once, the difficulty of obtaining high grade optical performance throughout the extended zooming range as well as the entire focusing range increases rapidly.
There is U.S. Pat. No. 4,629,294 as a related art of the Invention.